The fabrication of waveguide structures for guiding electromagnetic energy, particularly in optoelectronic type devices, is of considerable interest. Optical interconnections offer advantages over their electrical counterparts, such as large signal bandwidths and reduced propagation delay. When circuit arrays are formed at the wafer scale level of integration, the advantages of optical interconnections become even more pronounced.
Much of the prior work which has been done on optical waveguide structures utilizes silica as the guiding medium. Silica is highly transmissive in the infrared region between 1.2 and 1.6.mu., which makes it an attractive medium for coupling to existing single mode optical fibers. In addition, processing techniques for silica structures based upon integrated circuit fabrication technologies are well understood.
Polymeric waveguide systems have also been studied in the prior art. Those studies have included consideration of low loss multilayer integrated optical waveguides using optically transparent polyimide as an embedding material and as a waveguide dielectric. Polymer-based structures also lend themselves well to many integrated circuit processing techniques and may be custom tailored for specific applications.
Aspects of semiconductor fabrication processes which are particularly of interest in the formation of optical waveguide structures are discussed in U.S. Pat. No. 4,912,022 (1990) to Urquhart et al., and U.S. Pat. No. 4,908,333 (1990) to Shimokawa et al. The type of sloped profile disclosed in U.S. Pat. No. 4,912,022 is stated to frequently exhibit a substantially Gaussian shape which is related to the random nature of the scattered radiation used to expose the resist.
There are five major groups of relevant processing steps: metal deposition upon substrates and wafers, wet etch of sputtered metals, thick- and thin-coated photolithographic graphic patterning, spin coating and ultraviolet light ("UV") cure of acrylic materials, and dry etch of acrylic materials using a reactive ion etch ("RIE") plasma.
Selvaraj et al., in, "Integrated Optical Waveguides in Polyimide for Wafer Scale Integration", IEEE Journal of Lightwave Technology, Vol. 6, No. 6, June 1988, pp. 1034-1044, disclose ion milling techniques for constructing mirrors and vertical vias enabling optical interconnection between layers of a multi-layer wafer scale structure.
Kokubun et al., in "Silicon Optical Printed Circuit Board for Three Dimensional Integrated Optics" in Electronics Letters, Vol. 21, No. 11, (1985), pp. 508-9, disclose forming a Vee-groove in a (100)-oriented crystalline silicon substrate by use of a sputtered silicon dioxide mask and a photomask of stripes aligned parallel or perpendicular to the &lt;111&gt; crystalline direction. An orientation dependent wet etch process creates Vee grooves. The grooves are cladded with a silicon dioxide film deposited by RF sputtering, and gold film is deposited on the (111) plane at the end of the Vee groove to enhance optical reflection. A mixture of styrene and benzyl methacrylate monomers is placed into the Vee grooves and cured to form the waveguide core. The authors also proposed trying unidirectional dry-etching techniques, such as reactive ion etching, in order to form reflectors with 45 degree angles to facilitate stacking of the plural substrates in their proposed approach.
Hartman et al., in "Radiant Cured Polymer Optical Waveguides On Printed Circuit Boards for Photonic Interconnection Use, Applied Optics, Vol. 28, No. 1, January, 1989, pp. 40-47 describe fabrication and evaluation of patterned channel waveguides formed on printed circuit card material by use of ultraviolet cured adhesive films as channel waveguide material and substrate patterning with standard photographic masks. Since the waveguide materials are formulated as adhesives, adhesion to the substrate was said not to be problem so long as a complete curing of the films occurs and film thickness does not exceed 250 .mu.. Waveguide patterning on silicon or glass substrates using an argon-ion laser is also discussed in this article. Channel waveguide widths of 3,200.mu. are proposed as a reasonable target or goal, suggesting formation of large, multimode optical waveguide structures.
Sullivan et al., in "Guided-wave Optical Interconnects for VLSI Systems", SPIE Vol. 881 Optical Computing and Nonlinear Materials, 1988, pp. 172-176 describe low loss optical interconnects made from polyimides. A direct write technique using negative-acting photosensitive polyimide is proposed to improve the smoothness of the resultant sidewalls. The authors set forth the principal requirements of an integrated waveguide technology in these terms: "low-loss signal propagation (&lt;0.1 dB/cm), simple and reproducible fabrication over very large areas (&lt;20 in.sup.2), easy fabrication of all routing and distribution circuit components, environmental stability (temperature, humidity and radiation), compatibility with multichip (MCP) packaging and printed wire board (PWB) fabrication, easy coupling at all interfaces (emitter/detector, fiber, waveguide-waveguide), and ability to transmit high power without channel degradation."
Sullivan, in "Optical Waveguide Circuits for Printed Wire-Board Interconnections", SPIE Vol. 994 Optoelectronic Materials, Devices, Packaging, and Interconnects II, (1988) pp. 92-100, discloses optical and electrical signal paths provided on the same substrate. The optical waveguides are formed by optical quality polyimide covered with a silicon dioxide cladding. Reactive ion etching through a two layer mask of photoresist and plasma-deposited silicon dioxide is used to form the waveguide channel. Right angle bends, branches, and crossover components for signal routing and distribution are also discussed.
Grande, et al., in "One-Step Two-Level Etching Technique for Monolithic Integrated Optics", Applied Physics Letters. Vol. 51, No. 26, Dec. 28, 1987, pp. 2189-2191, propose the use of an erodable mask to produce both shallow etching and deep etching of the substrate in order to accommodate dissimilar optical devices on the same substrate.
In the fabrication of polymer based channel waveguides and rib waveguide structures, it is advantageous and often necessary to have extremely thin polymer films incorporated into the completed structure. Various semiconductor-type processing methods may be used for creating thin films out of the materials which are used to make polymer resin based waveguide structures. One method is spin-coating, in which a fluid polymer precursor resin material is dispensed onto a wafer. The wafer is spun at a rotational speed and for a duration selected to yield a desired film thickness. Another method of creating thin films is plasma etching. For organic materials, an oxygen plasma applied under isotropic conditions is used to reduce evenly a thick polymer film to a desired thickness.
Spin-coating and oxygen plasma etching have been combined when it is desired to planarize a rough or uneven surface. In one case a film of appropriate thickness is spin-coated onto a rough surface. Providing the etch rates of the underlying layer and the spin-coated layer are equal, the isotropic oxygen plasma may then be used to reduce uniformly the overall film thickness, leaving behind a smoothly surfaced (planarized) film of the underlying material. In another case, when a thin spin-coated film will not planarize an uneven surface, a very thick film may be spin-coated onto the wafer and then reduced to the required thickness by using an isotropic oxygen plasma.
Materials for forming the channel waveguide media must satisfy a number of requirements. Among the properties such materials must possess are: high optical transparency at the wavelengths of interest (especially 550-1550 nm), rapid and complete cure characteristics, workable fluid phase precursor consistencies prior to placement and cure, and selectable/controllable refractive indices. These materials must also have the property of adhering securely to substrates, such as polyimide, gallium arsenide, indium phosphide, silicon nitride, and crystalline silicon. They must also show good interlayer adhesion. Ultraviolet light curable polymers have been proposed in the prior art for channel waveguide structures.
Heretofore, the adhesion issue has been problematic. Not only is it difficult to adhere to the various substrates noted above, but interlayer adhesion between ultraviolet light curable layers is generally recognized as a major problem. Thus, a hitherto unsolved need has remained for fabrication methods, conditions and materials which manifest the requisite physical and optical properties while also manifesting greater interlayer adhesion and greater adhesion to difficult substrates.
The present invention provides significant and unexpected improvements applicable to this technology, particularly in the formation of very narrow, single optical mode waveguide structures and out-of-plane mirrors.